resetting k3s in NixOS is not that straightforward and requires manual input. It cannot be fully automated as removing the statement from your config afaik unless this has changed recently.
Now I don't recall which issue I had, I think it was something related to CoreDNS config or passing through the /etc/hosts of the NixOS node, but I do remember having to touch the K3s YAML directly, and maybe having issues also persisting it. It's actually the only thing I fear would break if I had to reinstall NixOS from scratch...
